Chemical reactors to at least partially convert a reactant flow stream into a product flow stream by promoting chemical reactions between the constituents of the reactant flow stream are known. The reactant flow stream is typically pre-conditioned to a suitable temperature and pressure for favoring the desired chemical reactions, and is subsequently routed through the chemical reactor, wherein the reactants are placed into contact with suitable catalysts to promote those chemical reactions.
These chemical reactions are oftentimes exothermic, although in some cases the reactions can be endothermic. At the same time, the reactions are often highly sensitive to the reaction temperature. It can be beneficial for the heat that is produced (or consumed, in the case of endothermic reactions) by the reaction to be removed from (or provided to, in the case of endothermic reactions) the reaction surfaces directly, rather than in a separate heat exchanger downstream of the reactor. The benefits that can be achieved by doing so include higher throughput of reactants and better selectivity of the reaction, among others.
A prime example of such a chemical reaction is the Fischer-Tropsch reaction, whereby carbon monoxide and hydrogen are reacted over a catalyst (typically Cobalt) to produce long hydrocarbon chains, which can subsequently be turned into liquid fuel. This reaction is exothermic, and heat needs to be removed from the reaction as it occurs in order to maintain a constant temperature. At large scales, Fischer-Tropsch reactors are usually slurry bubble reactors, wherein the catalyst consists of small particles suspended in a slurry. The gaseous reactants bubble up through the slurry, reacting as the bubbles contact the catalyst.
Due in part to the decreasing costs of natural gas (which can provide the feedstock chemicals for the reactions) coupled with the increasing cost and scarcity of liquid fuels, the desirability of producing fuel via the Fisher-Tropsch process is increasing. However, oftentimes the production rate of the feedstock fuel is insufficient to support the large cost associated with conventional, large scale Fisher-Tropsch reactors. Transporting the natural gas feedstock to a centralized location for processing is often impractical or economically prohibitive. As a result, much attention is now focused on process-intensified reactors to enable the economic production of Fischer-Tropsch fuels at a substantially smaller scale than can be achieved using the aforementioned conventional technology.
While such small scale Fischer-Tropsch reactors have been demonstrated, there is still much room for improvement. Such small scale reactors typically include long channels containing catalyst, through which the reactants flow and react to form a paraffinic wax. The reactant flow is typically in the direction of gravity, so that the wax can drip down out of the channels. It is preferential to have many channels with small hydraulic diameters in order to decrease the resistance to mass transfer and heat transfer. A coolant (boiling water, for example) flows through coolant channels in the reactor in order to absorb the heat of reaction and maintain a near-isothermal temperature.
One challenge in constructing such a small-scale reactor is that it can be difficult to place the required catalyst in a long channel. The typical approach is to apply the catalyst as a coating to inserts that are then placed into the long channels. Such an approach is described in U.S. Pat. No. 7,749,466 and US patent application 2010/0324158. This approach has several disadvantages, including the need to handle multiple parts (thereby adding cost), as well as the lack of good thermal contact between the catalyzed surfaces and the channel surfaces through which the heat must be transferred.
An alternative approach is disclosed in U.S. Pat. No. 8,278,363. In that approach, the reactor includes multiple compact finned-tube heat exchangers, each of which are coated with catalyst on the external finned surfaces. The heat exchangers are arranged in series with respect to the reactant flow, and cooling water is separately plumbed to each heat exchanger. However, since the depth to which the catalyst coating can be applied is relatively shallow, and the flow length is relatively long, this approach can require a great number of heat exchangers. For example, the depth to which the catalyst coating can be applied is typically in the range of 1-2 inches, whereas the desired flow length is typically in the range of 30-50 inches.